When it comes to heavy hauling, most people would agree that the Ford Super Duty platform is probably the most favored of the bunch. Since its inception, annual sales have been staggering at over 200,000 per year. This equates to well over a million Super Duty trucks in service today. The bulk of these were equipped with the 7.3L Power Stroke diesel option, and the great majority of these came with the time-tested four-speed automatic known as the E40D or 4R100. This transmission was suitable for stock power levels (and end users who do things by the book), however, once overloaded or exposed to increased torque loads, both the E40D and 4R100 quickly showed signs of weakness.
That's why we did a little investigative work to see how these transmissions could be configured to perform reliably under even the most demanding conditions. Our search led us to John Wood Automotive of Holtville, California. Wood started building Ford transmissions in 1983 and prides himself with a reputation for utmost attention to detail and customer satisfaction. Wood operates his business on the principle of quality before quantity, which with only one assistant requires he take on only one transmission rebuild per day. This methodology pays off, as most of Wood's patrons are referred by word of mouth. Diesel drag racing, sled pulling, sand-dune running, and heavy towing are the primary lifestyles of Wood's customers.
Wood has an intensive educational rebuild program that's set up to teach Super Duty owners all the ins and outs of their transmission in one full eight-hour work day. Some customers take notes while others take a rain check and visit the nearby Glamis OHV area, but everybody leaves happy knowing they have the most efficient, reliable, and robust E40D or 4R100 money can buy.
1. Prior to teardown, John conducts a one-on-one interview with each customer to determine exactly how crazy his or her transmission needs are. Ours included heavy towing, occasional sled pulls, and a 50-mile daily commute in stop-and-go traffic. | 
2. The first thing Wood explains to customers is where Ford went wrong when they designed the transmission in the first place. Notice the small "bypass" line on the workbench. This line was designed to allow transmission fluid to reach operating temperature quickly after start-up. Basically, it involves an inline check valve that opens up, allowing ATF to bypass the cooling circuit when restrictions are above a specific value, e.g., cold temperatures, or with the addition of extra transmission coolers and/or increased line pressure. This is bad for those of us who like to improve things over stock. Without proper cooling, ATF quickly degrades, causing internal components to wear prematurely. | 
3. One look at the magnet located inside the bottom of the pan confirmed that our donor's transmission had been through the wringer. Excessive heat causes internal clutches and other parts to break down. |
4. Upon teardown, Wood showed us several worn-out clutch materials. This transmission failed because the owner had recently added an aftermarket programmer designed to increase power over stock. This added power increased the transmission's line pressure, which in turn caused the bypass valve to open up, eliminating the cooling circuit. This quickly caused the transmission to build up excessive heat each and every time the truck was driven. | 
5. Wood recommends a simple solution to prevent this problem from killing your Super Duty transmission; braze up the small holes in the banjo bolt as seen here. He cautioned us that this procedure must be executed without restricting the center flow-through of the bolt. By closing off these four holes, the ATF bypass tube is eliminated from the system, ensuring the ATF will always go through the cooling circuit. | 
6. Wood also offers an inexpensive aluminum block-off spacer for those who don't feel confident brazing the holes up themselves. |
7. Next, Wood torqued down the center support bolts of the integrated valve body to 14 lb-ft in preparation for resurfacing. These bolts almost always work themselves loose over time, causing internal leakage and low ATF line pressure. Once tightened appropriately, Wood block-sanded the valve-body sealing surface. This resurfacing ensures the valve body will not leak once the transmission is rebuilt. Once perfect, the remaining internals of the transmission were removed from the case and the case was placed in a hot solvent bath. | 
8. While the case was being cleaned, Wood showed us some of the various parts he uses to firm up shifts inside the 4R100 valve body. Depending on customer needs, Wood replaces valve-body internals with parts from TransGo, as well as a few custom machined items not shown. | 
9. Here are two very different converter clutch pistons. The top piston is the OE stamped-steel piston. Below is Wood's custom-made billet piston. Aside from sealing better, this improved piston will not warp or flex. This little touch helps apply equal pressure to the torque-converter clutch, which also helps prevent torque-converter failure. |